I had Marcel neutered at 10 mths old, partly bc I wanted him to weigh as much as possible, partly bc he was still healing from giardia. He was just about 3lbs when neutered, and he did *great*.
Spaying is a more involved surgery, but there are plenty of members here who've had their girls spayed and only weighed 3lbs or less. Maybe your vet is overly cautious (not a bad thing
)....but I think it's sort of your call too, if you feel this is the right time for her to be spayed. Is your vet used to spaying smaller dogs?
I'm sure she'll do just great, Mike.